1. Field of the Invention
The present invention relates to a method for manufacturing a thin film transistor and a method for manufacturing a display device including the thin film transistor.
2. Description of the Related Art
In recent years, thin film transistors that are formed using a semiconductor thin film having a thickness of several nanometers to several hundreds of nanometers over a substrate having an insulating surface such as a glass substrate have been attracting attentions. Thin film transistors are widely used for electronic devices such as ICs (integrated circuits) and electro-optical devices. In particular, thin film transistors are urgently developed as switching elements of image display devices typified by liquid crystal display devices, EL (electro-luminescent) display devices, and the like. In an active matrix liquid crystal display device, for example, a voltage is applied between a pixel electrode connected to a selected switching element and an opposite electrode corresponding to the pixel electrode, and thus, a liquid crystal layer disposed between the pixel electrode and the opposite electrode is modulated optically. The optical modulation can be recognized as a display pattern by an observer. Here, the term “active matrix liquid crystal display device” refers to a liquid crystal display device which employs a method in which a liquid crystal layer is modulated optically by driving pixel electrodes arranged in matrix using switching elements, so that display patterns are formed on a screen. The term “active matrix EL display device” refers to an EL display device which employs a method in which a display pattern is formed on a screen by driving pixels arranged in matrix using switching elements.
The range of application of the active matrix display devices is expanding, and demands for larger screen size, higher definition, and higher aperture ratio are increasing. In addition, it is demanded that the active matrix liquid crystal display device has high reliability and that a method for manufacturing the active matrix liquid crystal display device offers high productivity and low manufacturing cost. As a method for increasing productivity and reducing manufacturing cost, simplification of the process can be given.
In an active matrix display device, thin film transistors are mainly used as switching elements. In manufacturing thin film transistors, reduction in the number of photomasks used in photolithography is important for simplification of the process. If one photomask is added, the following steps are further needed: resist application, prebaking, light exposure, development, postbaking, and the like, and moreover other steps before and after the above-described steps, such as film formation and etching and further resist removal, cleaning, drying, and the like. The number of steps is significantly increased only by adding one photomask in the manufacturing process. Therefore, many techniques for reducing the number of photomasks in a manufacturing process have been developed.
Thin film transistors are broadly classified into a top gate type in which a gate electrode is formed over a channel formation region and a bottom gate type in which a gate electrode is formed under a channel formation region. It is known that the number of photomasks used in a manufacturing process of bottom-gate thin film transistors is less than the number of photomasks used in a manufacturing process of top-gate thin film transistors. Bottom-gate thin film transistors are generally formed using three photomasks.
Many conventional techniques for reducing the number of photomasks use a complicated technique such as backside light exposure, resist reflow, or a lift-off method, which requires a special apparatus. There has been a contributing factor that yield is reduced due to various problems caused by the usage of such a complicated technique. Moreover, there is often no option but to sacrifice electrical characteristics of thin film transistors.
As a typical method for reducing the number of photomasks in a manufacturing process of a thin film transistor, a technique using a multi-tone mask (called a half-tone mask or a gray-tone mask) is widely known. As a technique for reducing the number of manufacturing steps by using a multi-tone mask, Patent Document 1 (Japanese Published Patent Application No. 2003-179069) can be given as an example.
However, even when bottom-gate thin film transistors are manufactured using a multi-tone mask as described above, at least two photomasks are needed, and it is difficult to further reduce the number of photomasks. One of them is used for patterning a gate electrode layer.